Blood pump with separate mixed-flow and axial-flow impeller stages and multi-stage stators

ABSTRACT

A pump for a fluid which can be blood has a stator housing and a rotor hub with leading and trailing portions and an intermediate portion disposed therebetween. At least one impeller blade at the leading portion drives circumferential and axial components of a flow into a pump annulus or intermediate pathway portion. At least one stator blade extends radially inward from the stator housing within the intermediate pathway portion and is configured to reduce a circumferential component of the flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/277,378 filed on May 14, 2014, which application claims the benefitof the filing date of U.S. Provisional Patent Application No. 61/823,224filed May 14, 2013, the disclosures of which are incorporated byreference herein.

BACKGROUND OF THE INVENTION Technical Field

This invention relates generally to pumps. More specifically, thisinvention relates to blood pumps, such as cardiac assist pumps that maybe implanted in a patient.

Description of the Related Art

Rotordynamic pumps, such as centrifugal, mixed-flow, and axial-flowpumps with mechanical bearings or magnetically suspended systems, havebeen widely used as a ventricular assist devices to support patientswith heart diseases. In magnetically levitated blood pumps, whichgenerally include an impeller that is both magnetically suspended androtated without mechanical means, an annular gap located between therotor and stator suspension and drive components is conventionallydesigned to be relatively small. A narrow annular flow gap generallynecessitates higher rotational speeds of the rotor in order to generatethe desired pressure rise and flow rates needed to support patients. Onechallenge of operating a rotor at high rotational speeds is a tendencyfor high turbulence flow characteristics within the blood (e.g., highshear stress) that can increase the extent and rate of red blood celldamage.

Additionally, for centrifugal or mixed-flow blood pumps with shroudedimpellers (i.e., a circumferentially revolved surface interconnectingthe impeller blade tips), the fluid within the clearance space between arotating front shroud and the stationary housing demonstrates a complexthree-dimensional structure, leading to retrograde leakage flow andstrong disk friction loss. The combination of disk friction loss and thestrong vortical flow can lower pump efficiency and in some cases carrygreater risks of hemolysis and thrombosis. A similar flow pattern canalso occur at the back clearance space between a rotating back shroudand the stationary housing for centrifugal or mixed flow pumps with orwithout a front shroud. The level of shear stress within the clearancebetween the walls of a shroud and housing depends, at least in part, onthe pump rotational speed.

For centrifugal or mixed-flow blood pumps with unshrouded or semi-openimpellers, the lack of a front shroud introduces a problem due to theblade tip leakage flow from pressure-side to suction-side of the bladeswhich occurs through the clearance between the rotating blade tip andthe stationary housing. The leakage flow can also generate a jet leakagevortex that interacts with the primary flow, causing hydraulic loss andpossibly inducing blood trauma. The shear stress exhibited in the gap orclearance between the blade tip gap and the stationary housing is verysensitive to the pump rotational speed as well as the magnitude of thegap itself.

For axial flow blood pumps with completely magnetically suspendedsystems, the annular gap located between the cylindrical rotor andhousing has to be small enough to maintain the magnetic radialstiffness. Additionally, the axial length of the rotor has to be sizedto maintain proper stability, exhibiting sufficient axial stiffness andlittle yaw. Such an arrangement generally leads to the requirement forhigh pump speed in order to generate the required pressure rise and flowrate for patients. However, the shear stress exhibited by the fluidwithin the annular gap region can become very high due to the highrotational speed and the narrowness of the gap. Moreover, conventionaldesigns of axial blood pumps tend to have very long blade profiles(i.e., extending long axial distances and having very large blade wrapangle) and large trailing edge angles (i.e., β2 close to 90 degrees).Such a design with very long blade profiles not only increases the bladetip areas with higher shear stress but also leads to flow separation andvortices, particularly at the off-design conditions.

In view of the foregoing, further improvements in rotordynamic pumps canbe provided.

SUMMARY OF THE INVENTION

Various embodiments of rotordynamic pumps for fluids are set forthherein in accordance with the present invention.

Exemplary embodiments may provide an apparatus and method for amultistage fluid pump for pumping a fluid such as blood or other fluid,in which a pump has a rotor hub having leading and trailing portionsadjacent an inlet and an outlet of the pump, respectively, and anintermediate portion between the leading and trailing portions. A rotorstage comprising at least one impeller blade is positioned at theleading portion of the pump. A stator stage comprising at least onestator blade extends radially inward from a stator housing in a portionof a fluid pathway that surrounds the intermediate portion and isconfigured to reduce a circumferential component of a flow. As usedherein, “radial” or “radially” mean in a radial direction away from arotational axis of the pump. An axial dimension of the at least onestator blade is smaller than a diameter of the intermediate portion ofthe rotor hub. As used herein, “axial” dimension means a dimension alongor parallel to the pump's rotational axis. A reduced axial dimension ofthe stator blade reduces contact between the stator blade and componentsof a fluid, such as red blood cells, for example, and may help reducerisk of damage to red blood cells over time. A second stator stagecomprising at least one second stage stator blade may be positioned in atrailing pathway portion of the fluid pathway that surrounds thetrailing portion of the rotor, and may reduce a circumferentialcomponent of a flow. In a particular example, a transitional outflowregion of the stator housing encompassing the trailing pathway portionmay define an interior conical space and the at least one second stagestator blade can extend inwardly into the conical space.

High efficiency, low blood damage, and small compact size are oftendesirable features for a long-term implantable blood pump. A reductionin the size of the pump may be facilitated by a stator blade positionedwithin an intermediate fluid pathway portion for reducing acircumferential component of the flow, thereby improving a rotationalstability of the pump, and possibly allowing the pump to operate withsmaller, more lightweight, or less complicated bearings. A reduction inrisk of blood cell damage may be facilitated by a stator bladeconfiguration in which each stator blade has a smaller axial dimensionthan heretofore contemplated, such as an axial dimension that is lessthan a diameter of an intermediate portion of the rotor hub. Red bloodcell damage in blood pumps is mainly related to the shear stress anddegree to which the red blood cells contact other surfaces such asimpeller blades and stator blades when passing through the flow paths.

Efficiency may also be improved when fluid is directed through a firststage rotating mixed-flow type impeller to gain both kinetic energy andpressure rise and then further to gain kinetic energy and pressure risefrom passage through second rotating axial impeller region after passagethrough the stator blade region. Such operation may yield a total higherhead (i.e., pressure rise) at the same pump speed than a single stagemixed-flow or single axial flow configuration thus resulting inincreased pump efficiency. Alternatively, it may be possible to operatethe multiple impeller stage pump at a lower speed and produce the samepressure rise as a single impeller stage configuration. The higherefficiency provides the benefit of low temperature rise of the motor andlonger battery life. As contact with bodily tissues is inherent to thedevice, the reduction in operating temperatures can reduce risksassociated with contact to surrounding body tissues. In addition, highershear stress regions in blood pumps usually occur in the blade tip gapregions, which are directly related to the pump speed. A two-stageimpeller design requires a lower pump speed than a purely single stagemixed-flow or axial flow blood pump in order to generate about 150 mmHgpressure rise for the need of a human body.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a longitudinal cross-sectional (meridional) view of a pump inaccordance with an embodiment of the invention:

FIG. 2 is a perspective view of various components of the pump shown inFIG. 1;

FIG. 3 is a perspective view of various components of the pump shown inFIG. 1 including with a partial cross-sectional view of a housingmember;

FIG. 4 is a longitudinal cross-sectional (meridional) view showing aconfiguration of a pump in accordance with another embodiment of theinvention;

FIG. 5 is a perspective view of various components of the pump shown inFIG. 4;

FIG. 6 is a perspective view of various components of the pump shown inFIG. 4 including with a partial cross-sectional view of a housingmember;

FIG. 7 is a longitudinal cross-sectional (meridional) view furthershowing a pump in accordance with a variation of the embodiment of theinvention illustrated in FIGS. 1, 2 and 3;

FIG. 8 is a longitudinal cross-sectional (meridional) view of a pump inaccordance with further embodiment of the invention;

FIG. 9 is a longitudinal cross-sectional (meridional) view showing aconfiguration of a pump in accordance with yet another embodiment of theinvention;

FIG. 10 is a longitudinal cross-sectional (meridional) view furthershowing a pump in accordance with the embodiment illustrated in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

For purpose of illustration, discussions of the technology will be madein reference to its utility as a cardiac assist blood pump. However, itis to be understood that the technology may have a variety of wideapplications to many types of turbomachinery including, for example,commercial and industrial pumps, compressors, and turbines.

Referring to FIGS. 1 through 3, a rotordynamic blood pump 100 is shownin accordance with an embodiment of the present invention. FIG. 1 showsa meridional section of the pump 100. FIG. 2 shows a perspective view ofvarious components of the pump, the housing of the pump being removedfrom the view for purposes of clarity. FIG. 3 shows a perspective viewof the pump 100 with the housing being sectioned to provide context toother components of the pump 100. Aspects of rotordynamic blood pumpsare disclosed in co-pending U.S. patent application Ser. No. 13/275,912filed Oct. 18, 2011, and Ser. No. 13/276,009 filed Oct. 18, 2011, thedisclosures of which are incorporated herein by reference.

The pump 100 includes a stator housing 102 having an inlet 104 and anoutlet 106. A rotor hub 108 having a generally cylindrical configurationis disposed within an interior volume defined by the housing such that afluid pathway includes an intermediate pathway portion 110 which definesan annulus or annular gap (“annulus” and “gap” also referenced herein by“110”) surrounding the rotor hub 108 within the stator housing 102. Therotor hub 108 includes a leading portion 112 (i.e., leading with respectto intended fluid flow through the pump 100), that may exhibit agenerally conical geometry and that is positioned near the inlet 104.Additionally, the rotor hub 108 includes a trailing portion 114 (i.e.,trailing with respect to intended fluid flow through the pump 100) thatexhibits a generally conical geometry and that is positioned near theoutlet 106. An intermediate portion 113 of the rotor hub extends betweenthe leading and trailing portions 112, 114 of the rotor hub. Theintermediate portion 113 may be substantially cylindrical in shape,having a diameter extending through an axis of the rotor hub. Theleading portion 112 may have a diameter which increases with a distancefrom the inlet to a diameter of the intermediate portion 113. Thetrailing portion 114 may have a diameter which decreases with proximityto the outlet 106; i.e., the diameter of the trailing portion maydecrease with a distance from the intermediate portion 113.

The pump 100 is configured with one or more impeller blades 116associated with the first, mixed-flow stage which can be formed on, orotherwise coupled with, the rotor hub 108 along the leading portion 112(i.e., in the conical region). Impeller blades 116 are positioned withina leading pathway portion of the fluid pathway in a space between theleading portion of the rotor hub 108 and the stator housing 102,providing suction to the fluid entering the inlet 104 and delivering thefluid in both an axial and a radially outward direction into anintermediate pathway portion 110 of the pump. The flow driven by theimpeller blades 116 into the intermediate pathway portion has an axialcomponent in a direction parallel to a rotational axis 125 of the rotorhub, and also has a substantial circumferential component in a directionof a circumference of the intermediate portion 113 of the rotor hub.

In the embodiment shown in FIG. 1, impeller blades 116 can beunshrouded. An unshrouded configuration may provide savings in cost andalso reduce the complexity of manufacturing such a pump. However, inother embodiments shrouds may be incorporated into the impeller designs.In an unshrouded configuration, a gap or clearance is maintained betweenlengthwise upper surface of the rotating impeller blades and thestationary stator housing.

As seen in FIG. 1, the intermediate portion 113 of the rotor hub has anaxial dimension 134 which is greater than a diameter 130 of theintermediate portion. Axial dimension is also referred to herein as“meridional length.” The axial dimension 134 corresponds to a totalannular gap length, i.e., length of the gap 110 between the intermediateportion 113 of the rotor and an inner wall of the stator housing 102 inan axial direction of the rotor hub. In accordance with one embodimentof the invention, pump 100 can be configured with magnetic bearingswhich eliminate the need for a mechanically bound and lubricated centralshaft on which the rotor hub rotates during operation. Rather, magneticbearings have a set of opposing rotor and stator magnets at positionsnear the ends of the rotor hub (i.e., ends corresponding to the leadingand trailing portions 112, 114) which magnetically suspend the rotor hub108 within the fluid pathway within the stator housing 102. Duringoperation of the pump, the magnets repel each other to maintain therotational axis 125 of the rotor hub 108 in a stable radial positionwithin the stator housing 102.

The circumferential component of the flow within the intermediatepathway portion 110 of the pump produces whirl forces which cannegatively impact the rotational stability of the pump 100, particularlya pump having magnetic bearings. The inventors have found that highwhirl forces can have a destabilizing effect on rotation of a rotor,which if unchecked could cause the rotational axis of a magneticallysuspended rotor to whip and result in touchdown of the rotor. High whirlforces can be overcome by providing larger magnetic bearings, but largermagnetic bearings typically increase the size and weight of the pumpwhich is less desirable from a surgical perspective.

It is noted that both the radial clearance and the axial dimension 134of the intermediate pathway portion 110, also referred to herein asannulus or annular gap can have a significant effect on pump performanceand possible blood damage. For a magnetically suspended and rotatedblood pump, the sizing of the annulus also has an effect on the radialand yaw stiffness of the suspension system. From a view point ofhydrodynamics, the radial gap (i.e., dimension of the annulus in theradial direction) the annulus should be made as large as reasonablypossible, while for the consideration of magnetic suspension system, theradial gap of the annulus should be small enough, and the axial lengthof the annulus should be long enough, to maintain a stable rotation ofthe rotor hub 106 within the stator housing 102. Improper design of suchcomponents, including the size of the annulus and the flowcharacteristics of the fluid passing through the annulus can lead to therotor hub 106 being unstable and exhibiting, for example, a whipphenomenon as it rotates within the stator housing 102 when configuredas a magnetically suspended or “levitated” pump.

It is noted that the components of the pump 100 are shown in relativelysimplistic forms for sake of clarity in the associated description. Forexample, the magnetic and electronic components that might be utilizedin association with a magnetically levitated pump are not specificallyshown. However, one of ordinary skill in the art will recognize thatsuch components will be inherently placed in or adjacent to the statorhousing 102 and within the rotor hub 108 to provide such a magneticallylevitated and rotated pump. One example of a completely magneticallysuspended system associated with a pump is described in U.S. PatentApplication Publication No. 20110237863 entitled “Magnetically LevitatedBlood Pump With Optimization Method Enabling Miniaturization”, thedisclosure of which is incorporated by reference herein.

The inventors have discovered that whirl forces within the pump can bedecreased, and the rotational stability of the pump can be increased byconfiguring the stator with one or more stator blades 101 for reducing acircumferential component of a flow into the intermediate pathwayportion 110 of the pump from the one or more impeller blades 116 of theleading portion upstream therefrom. The stator blades 101, disposed onan inner surface of the stator housing 102 or mechanically coupled tothe stator, and projecting within the intermediate pathway portion 110,typically twist in a direction opposite that in which the impellerblades twist. In this way, the stator blades 101 help to recover kineticenergy of the fluid (e.g., blood) and lead the fluid to flow in a moreaxial direction through the pump towards the outlet 106.

A gap or clearance exists between the lengthwise lower surface of thestator blades 101 and the rotor hub 108. The extent of both the impellerblade 116 tip clearances and the stator blade 101 tip clearances canhave significant effects on the pump's performance including, forexample, pump head and efficiency. Additionally, these clearances canhave a significant impact on the amount of damage that may occur to theblood cells. In one particular embodiment, both the impeller blade tipclearances and the stator blade tip clearances may be approximately 0.1mm to approximately 0.2 mm. However, the clearances may be set at otherdistances depending on a variety of factors.

In one embodiment, the stator stage can be disposed within theintermediate pathway portion 110 of the pump at a position as close aspossible to the leading portion 112. In this way, whirl forces generatedby a circumferential component of the flow coming off of the rotor stagecan be reduced closer to an entrance within the intermediate pathwayportion 110. In one embodiment, an axial dimension 132 of the at leastone stator blade 101 can be any length from about 2% to 98% of the totalannular gap length in the axial direction. Stator blades which arelonger and extend to greater meridional lengths can have greaterefficiency in reducing whirl forces surrounding the intermediate portionof the rotor hub, and in recovering pressure to increase the pump headand efficiency. In a pump 100 as seen in FIGS. 1-3, an axial dimensionof the intermediate portion is greater than a diameter 130 of theintermediate portion of the rotor hub 108. The inventors have found thatreducing the axial dimension 132 of the one or more stator blades 101 toa value that is less than the diameter 130 of the intermediate portion,this can help reduce damage to blood during operation of the pump.

Downstream of the first stage stator blades 101, adjacent the pumpoutlet 106 and the trailing portion 114 of the rotor hub 108, one ormore second stage stator blades 120 can extend from an inner surface ofthe stator housing 102. The stator blades 120 help to recover kineticenergy of the fluid (e.g., blood) and lead the fluid to flow axiallythrough the outlet 106. A gap or clearance exists between the lengthwiselower surface of the stator blades 120 and the trailing portion 114 ofthe rotor hub 108. The stator blades 120 also help to reduce turbulencethat might develop during transition of the flow from the annulus 110through the outlet 106. As with the stator blade 101 tip clearancesdescribed above, the extent of the stator blade 120 tip clearances canhave significant effects on the pump's performance including, forexample, pump head and efficiency. Additionally, these clearances canhave a significant impact on the amount of damage that may occur to theblood cells. In one particular embodiment, the clearances of each of theimpeller blade tips and the tips of the stator blades 101 and 120 may beapproximately 0.1 mm to approximately 0.2 mm. However, the clearancesmay be set at other distances depending on a variety of factors.

During operation of the pump, fluid enters through the inlet 104 of thepump 100 and encounters the first-stage impeller blades 116. Thepressure of the fluid is raised by the first-stage impeller blades 116and directed both radially outward and axially forward into theintermediate pathway portion 110 between the stator housing 102 and therotor hub 108. The fluid then encounters the stator blades 101 whichhelp to capture some of the kinetic energy of the fluid and direct thefluid in more of an axial direction of the pump 100.

After the fluid (e.g., blood) passes the stator blades 101 within theintermediate pathway portion as it travels towards the outlet, the fluidtends to regain momentum in a circumferential direction, due to rotationof the rotor hub 108 and a viscosity characteristic of the fluid. Thesecond stage stator blades 120 again help convert some of the kineticenergy of the fluid into pressure at the outlet 106, by reducing thecircumferential component of the momentum such that the fluid isdirected more in an axial direction of the pump. In this way, the fluidflowing through the outlet 106 is directed in more of an axial directionof the pump.

Still referring to FIGS. 1-3, it is seen that the pump may include three(3) first-stage impeller blades 116 of mixed-flow type, three (3)first-stage stator blades 101 of axial-flow type, and four stator blades120. Of course, it is contemplated that other arrangements having moreor fewer impeller blades 116 or stator blades 101, 120 may be utilized.The impeller blades 116 and the first and second stage stator blades101, 120 all have 3-dimensional curved surfaces which can be designed,for example, using conventional turbomachinery inverse design theorysuch as 2D or quasi-3D methods. Their shapes and numbers may also beoptimized via computational fluid dynamics (CFD) to reach the highestefficiency with minimal blood damage.

Another feature of the embodiment of FIGS. 1-3 is that pump operationcan be provided by the single-stage impeller blades 116, since no otherstage of impeller blades need be provided. Specifically, in theembodiment of FIGS. 1-3, there is no impeller blade in the intermediatepathway portion 110 between the first-stage stator blades 132 and thesecond-stage stator blades 120. In this way, the second-stage statorblades may help to reduce a circumferential component of a flow receivedfrom the first stator stage, wherein the flow is in a form undriven byan impeller blade between the first and second stator stages. Inaddition, with the pump shown in FIGS. 1-3, the flow from inlet 104 tothe outlet 106 may be driven substantially only by a single rotor stagehaving the at least one impeller blade 116. Downstream stator blades 101reduce a circumferential component of the flow as the fluid travelstowards the outlet. Second stage stator blades 120, if present, may alsohelp reduce a circumferential component of the flow as the fluid isdelivered to the outlet 106.

In one particular embodiment, the pump 100 may be configured as animplantable blood pump wherein the rotor hub 108 is magneticallysuspended and rotated. The rotor hub 108 may exhibit and overall lengthof approximately 106 mm and a diameter 130 of approximately 12.4 mm. Theinside diameter of the stator housing 102 may be approximately 16 mm,resulting in a clearance gap (for the intermediate pathway portion 110or annulus) of approximately 1.8 mm between rotor hub 108 and the innersurface of the stator housing 102. The inlet 104 and outlet 106 may eachexhibit a diameter of approximately 8 mm. In such an embodiment, it hasbeen calculated that blood entering the inlet 104 at a total pressure(i.e., kinetic pressure plus static pressure) of approximately 0millimeters of mercury (mmHg), and at a flow rate of approximately 5liters per minute (LPM), will experience a total increase of pressure ofapproximately 190 mmHg when it flows through the impeller blades 116with the rotor hub 108 rotating at a speed of approximately 16,000rotations per minute (RPM). Though the fluid experiences head loss as itflows through the remainder of the pump, the stator blades 101, 120 willhelp to capture kinetic energy and convert it into pressure while alsodirecting the flow of the fluid in a more axial direction and reducing acircumferential component of the flow. Thus, while the fluid pressuremay decrease as the fluid moves downstream from the impeller, the statorblades can serve to reduce turbulence such that the pressure of thefluid leaving the outlet 106 will be approximately 190 mmHg. Of course,such an example is not to be considered limiting in any sense. The pump100 may be configured to exhibit different dimensions, operate atdifferent rotational speeds, and process fluid at different flow ratesand pressures.

Referring to FIGS. 4-6, another example of a pump 200 is seen. In thiscase, a two-stage impeller configuration is employed which may includethree (3) first-stage impeller blades 216 of mixed-flow type, threefirst-stage stator blades 201, three (3) second-stage impeller blades203 of axial-flow type, and four stator blades 220. Of course, it iscontemplated that other arrangements having more or fewer impellerblades 216, 203 at each stage or fewer stator blades 201, 220 may beutilized. The shapes and numbers of the impeller blades and statorblades may also be optimized via computational fluid dynamics (CFD) toreach the highest efficiency with minimal blood damage. In one example,the mixed-flow first-stage impeller blades 216 and the axial-flowsecond-stage impeller blades 203 may be designed, with respect to thehead, such that the first stage provides approximately 50% toapproximately 70% of the total pump head, while the second stage mayprovide approximately 30% to approximately 50% of the total pump head.

The leading edge angle of stage-two impeller blades 203 along eachstreamline may be set to be approximately equal to the trailing edgeangle of the first-stage impeller blades 216 with a plus or minus attackangle of 0° to 5° by inverse design theory and CFD optimization inaccordance with the flow modified by stator blades 201 so that the flowfrom the stator blades 201 matches well with the leading edge of thesecond stage impeller blades 203. The second stage blades 203 may bedesigned by aerofoil cascade theory together with CFD optimization toavoid complex and unreasonable very long blades. The stator blades 220may be designed so that the leading edge angles generally match the flowout of the stage-two impeller blades 203. The trailing edge angles ofthe stator blades 220 may be approximately 90° so that the blood orother fluid can be led to the outlet substantially uniformly withoutminimal turbulence. The blade-to-blade sections and the meridionalsection part near the stator blades 220 (as depicted in FIG. 4) may bedesigned and optimized by CFD so that they can further recover somepotential energy (pressure) from the kinetic energy of the fluid flow.

It is noted that the second-stage impeller blades are positioneddownstream within the annulus 210 nearer to the second stage statorblades 220 than to the first-stage impeller blades 216. Stated anotherway, the second-stage impeller blades 203 are positioned nearer totrailing portion 214 of the rotor hub 208 than to the leading portion212 of the rotor hub 208 (and nearer to the inlet 204 than to the outlet206). The positioning of the second-stage impeller blades 203 nearer tothe trailing portion 214 may provide greater stabilization to the rotorhub 208 during operation of the pump 200 so as to minimize or preventany whip phenomenon that might occur. For example, because of theincrease in circumferential velocity of the fluid imposed by thesecond-stage impellers, when the second-stage impeller blades 203 arepositioned nearer to the leading portion, the rotor hub under certainoperating conditions may experience a whip phenomenon and exhibits signsof instability. In the embodiment shown in FIG. 6, the second-stageimpeller blades 203 can boost the pressure and flow rate (as with theconfiguration described with respect to FIGS. 1-3) while providingincreased stability because the increased circumferential velocityimparted by the second-stage impeller blades 203 will be converted,nearly immediately, into pressure by the stator blades 220 and thecircumferential velocity will be significantly reduced, to nearly zero,as the fluid flows out from the stator blades 220.

Referring again to FIG. 4, in another embodiment which is notspecifically shown therein, but which is a variation of theabove-described embodiment, the second-stage impeller blades 203 areomitted. Stator blades 201 are relatively long, having axial dimensions232 which are substantial in relation to an axial dimension 234 of theintermediate pathway portion 210 of the pump. Such stator blades 201,whose axial dimensions can be greater than the diameter 230 of theintermediate portion 213 of the rotor, and in some cases, greater thanhalf the axial dimension 234 of the intermediate portion 213, are longerthan the stator blades 101 in the embodiment described above withrespect to FIGS. 1-3.

In this variation, a flow from the inlet 204 to the outlet 206 of thepump is driven substantially only by impeller blades 216 of the rotorstage at the leading portion 212. Stator blades 201 which are relativelylong, i.e., longer than those of the FIGS. 1-3 embodiment, help tostraighten the flow through a longer portion of the intermediate pathwayportion 210 by reducing a circumferential component of the flow, and mayfurther help to reduce whirl forces within the pump 200. Optional statorblades 220, if present at the trailing portion, may further help tostraighten the flow, reducing a circumferential component thereof, asthe fluid leaves the intermediate pathway portion 210 and is deliveredto outlet 206.

Referring now to FIG. 7, a two impeller stage pump 300 is shown inaccordance with a variation of the embodiment described above withreference to FIGS. 4, 5 and 6, having a first stage of impeller blades316 and a second stage of impeller blades 303, and having a first stageof stator blades 301 and a second stage of stator blades 320. In thisvariation, an axial dimension 332 of the first-stage stator blades 301is smaller than a diameter 330 of the intermediate portion of the rotorhub 308. This aspect of pump 300 is more similar to the configurationseen in the embodiment described above with respect to FIGS. 1, 2 and 3which have a like configuration of stator blades 301. In this case, thetwo impeller-stage configuration can have increased pumping efficiency,which may facilitate lower speed operation relative to the configurationseen, for example, in the pump 100 of FIGS. 1-3. In addition, statorblades 301 having an axial dimension 332 shorter than the diameter 330of the intermediate portion may help to reduce contact of the blades 301with the blood and may help reduce damage to the blood.

FIG. 8 illustrates a further variation of the embodiment shown anddescribed above relative to FIGS. 1-3 in which additional stator blades440 or vanes can be provided within the inlet 404 of the pump, thesehelping to reduce pre-rotation for the blood before entering the leadingpathway portion surrounding the impeller blades 416. Such inlet blades440 can be of particular benefit in off-design conditions such as withvery small rate of flow through the pump. Additional blades 442 withinthe outlet 406 of the pump can also further help to straighten the flowfrom the blades 420 of the second stator stage to recover staticpressure at the outlet 406 of the pump 400.

FIG. 9 illustrates a further variation in which an additional inletstage of stator blades 440 and an outlet stage of stator blades 442,such as seen in FIG. 8, are provided in a pump having a configurationsuch as described above with respect to FIGS. 4-6.

FIG. 10 illustrates a further variation in which an additional inletstage of stator blades 440 and an outlet stage of stator blades 442,such as seen in FIG. 8, are provided in a pump having a configurationsuch as described above with respect to FIG. 7.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the inventionincludes all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims. It is specifically noted that any features or aspectsof a given embodiment described above may be combined with any otherfeatures or aspects of other described embodiments, without limitation.

What is claimed is:
 1. An implantable blood pump comprising: a statorhousing including an inlet and an outlet and defining a fluid pathwaytherebetween; a rotor hub disposed within the stator housing, the rotorhub defining a diameter and a rotational axis extending from the inletto the outlet of the stator housing; and a stator blade extending inwardfrom the stator housing and defining a length less than the diameter ofthe rotor hub.
 2. The blood pump of claim 1, further comprising an inletstator blade extending inward from the stator housing within the inlet.3. The blood pump of claim 2, wherein the rotor hub includes a bodyhaving a leading portion including a conical geometry proximate theinlet of the stator housing, a trailing portion proximate the outlet ofthe stator housing, and an intermediate portion extending between theleading portion and the trailing portion, the intermediate portion ofthe rotor hub defining an intermediate portion of the fluid pathway, andthe stator blade projects within the intermediate portion of the fluidpathway.
 4. The blood pump of claim 3, wherein the inlet stator bladedefines the fluid pathway from the inlet to the leading portion of thebody of the rotor hub, and the inlet stator blade is configured toreduce a pre-rotation of a fluid within the inlet.
 5. The blood pump ofclaim 4, wherein the inlet stator blade includes a distal-most portionfacing the rotational axis of the rotor hub.
 6. The blood pump of claim5, wherein the stator blade is disposed distally to the outlet withrespect to the inlet stator blade.
 7. The blood pump of claim 6, furthercomprising an impeller blade at the leading portion of the rotor hub,the impeller bade and the stator blade each defining a rotationalpathway opposite with respect to each other.
 8. The blood pump of claim1, further comprising an outlet stator blade extending inward from thestator housing within the outlet.
 9. The blood pump of claim 8, whereinthe outlet stator blade includes a distal-most portion facing therotational axis.
 10. An implantable blood pump comprising: a statorhousing including an inlet and an outlet and defining a fluid pathwaytherebetween; an inlet stator blade extending inward from the statorhousing within the inlet; and a rotor hub disposed within the statorhousing, the rotor hub defining a diameter and a rotational axisextending from the inlet to the outlet of the stator housing.
 11. Theblood pump of claim 10, wherein the inlet stator blade includes adistal-most portion facing the rotational axis of the rotor hub.
 12. Theblood pump of claim 10, further comprising a stator blade extendinginward from the stator housing and defining a length less than thediameter of the rotor hub.
 13. The blood pump of claim 12, wherein therotor hub includes a body having a leading portion proximate the inlet,a trailing portion proximate the outlet, and an intermediate portionextending between the leading portion and the trailing portion, theintermediate portion of the rotor hub defining an intermediate portionof the fluid pathway, and the stator blade projects within theintermediate portion of the fluid pathway.
 14. The blood pump of claim13, wherein the stator blade is disposed distally to the outlet withrespect to the inlet stator blade and adjacent the leading portion ofthe body of the rotor hub.
 15. The blood pump of claim 14, wherein therotor hub includes an impeller blade, the impeller blade and the statorblade each define a rotational pathway opposite with respect to eachother.
 16. The blood pump of claim 14, wherein the inlet stator bladedefines the fluid pathway from the inlet to the leading portion of thebody of the rotor hub.
 17. The blood pump of claim 10, wherein the inletstator blade is configured to reduce a pre-rotation of a fluid when inthe inlet of the stator housing.
 18. The blood pump of claim 10, furthercomprising an outlet stator blade extending inward from the statorhousing within the outlet.
 19. The blood pump of claim 10, furthercomprising a stator blade extending inward from the stator housing anddefining a length greater than the diameter of the rotor hub.
 20. Animplantable blood pump comprising: a stator housing including an inletand an outlet and defining a fluid pathway therebetween; a rotor hubdisposed within the stator housing, the rotor hub defining a diameterand a rotational axis extending from the inlet to the outlet of thestator housing and including a body having a leading portion including aconical geometry proximate the inlet of the stator housing, a trailingportion proximate the outlet of the stator housing, and an intermediateportion extending between the leading portion and the trailing portion,the intermediate portion of the rotor hub defining an intermediateportion of the fluid pathway; an inlet stator blade extending inwardfrom the stator housing within the inlet and defining the fluid pathwayfrom the inlet to the leading portion of the body of the rotor hub; anda stator blade extending inward from the stator housing, the statorblade defining a length less than the diameter of the rotor hub andprojecting within the intermediate portion of the fluid pathway.